The present patent application is related to solid state light emission devices.
Solid-state light sources, such as light emitting diodes (LEDs) and laser diodes, can offer significant advantages over incandescent or fluorescent lighting. The solid-state light sources are generally more efficient and produce less heat than traditional incandescent or fluorescent lights. When LEDs or laser diodes are placed in arrays of red, green and blue elements, they can act as a source for white light or as a multi-colored display. Although solid-state lighting offers certain advantages, conventional semiconductor structures and devices used for solid-state lighting are relatively expensive. The high cost of solid-state light emission devices is partially related to the relatively complex and time-consuming manufacturing process for solid-state light emission devices.
Referring to FIG. 1, a conventional LED structure 100 includes a substrate 105, which may be formed of sapphire, silicon carbide, or spinel, for example. A buffer layer 110 is formed on the substrate 105. The buffer layer 110 serves primarily as a wetting layer, to promote smooth, uniform coverage of the sapphire substrate. The buffer layer 310 is typically deposited as a thin amorphous layer using Metal Organic Chemical Vapor Deposition (MOCVD). An n-doped Group III-V compound layer 120 is formed on the buffer layer 110. The n-doped Group III-V compound layer 120 is typically made of GaN. An InGaN quantum-well layer 130 is formed on the n-doped Group III-V compound layer 120. An active Group III-V compound layer 140 is then formed on the InGaN quantum-well layer 130. A p-doped Group III-V compound layer 150 is formed on the layer 140. A p-electrode 160 (anode) is formed on the p-doped Group III-V compound layer 150. An n-electrode 170 (cathode) is formed on the n-doped Group III-V compound layer 120.
A drawback in the conventional LED devices is that different thermal expansions between the group III-V layers and the substrate can cause cracking in the group III-V layers or delamination between the group III-V layers from the substrate.
A factor contributing to complexity in some conventional manufacturing processes is that it requires a series of selective etch stages. For example, the cathode 170 in the conventional LED structure 100 shown in FIG. 1 is formed on the n-doped Group III-V compound layer 120 by selectively etching. These selective etch stages are complicated and time-consuming and, therefore, make the overall manufacturing process more expensive.
It is also desirable to increase active light emission intensities. The conventional LED device in FIG. 1, for example, includes non-light emission areas on the substrate 105 that are not covered by the InGaN quantum-well layer 130 to make room for the n-electrode 170. The p-electrode 160 can also block some of the emitted light from leaving the device. These design characteristics reduce the emission efficiency of the conventional LED devices.
Another requirement for LED devices is to properly direct inward-propagating light emission to the intended light illumination directions. A reflective layer is often constructed under the light emission layers to reflect light emission. One challenge associated with a metallic reflective layer is that the metals such as Aluminum have lower melting temperatures than the processing temperatures for depositing Group III-V compound layers on the metallic reflective layer. The metallic reflective layer often melts and loses reflectivity during the high temperature deposition of the Group III-V compound layers.